Method and apparatus for time-profiling T-carrier framed service

ABSTRACT

A method and apparatus for frame-level, sub-second error reporting is disclosed. Trouble profiles are compiled using synchronous constant-frame error detection and, during a transmission hiatus, pseudo-synchronous constant-frame error detection so as to describe transmission integrity down to the millisecond. This provides profiles that can be used to trace the cause(s) of outages to complex networks including outages from network activity. The outage profiles obtained from the described device can be used to determine and confirm the source of sub-second, or more, transmission outages.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional application No.60/113,929 filed Dec. 24, 1998, entitled Method and Apparatus forTime-Profiling T-Carrier Framed Service.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to the maintenance of communicationsnetworks. More particularly, the invention is directed to improvedmonitoring of transmissions errors in telecommunications networks.

2. Discussion of Related Art

It has been known to provide synchronous-transfer modes (STM) control oftraffic using frames for such services as T-1, T-3 and others. Morerecently, hybrid ATM/TDM networks provide synchronous-transfer mode(STM) control of traffic from both the ATM and TDM payload sources. Theypermit a graceful, piecewise implementation of the megabits-per-secondspeeds of asynchronous transfer mode (ATM) packet-routing segmentswithin existing voice, fax and data-communications networks havingconventional digitally-switched call circuits (DSCs) that carry STMcommunications. STM communications use a signal-reference frame thatprovides dedicated time slots for the time-division multiplexing (TDM)of signals passing over the individual links in an STM network.

In theory, it should be possible to use the TDM trouble shootingcriteria for monitoring the quality of hybrid ATM/TDM service and obtainthe level of quality and efficiency achieved in purely TDM networks.However, field experiments that attempted real-time detection oftransmission errors in call circuits combining ATM and TDM payloadsresulted in imprecise descriptions of the troubles. The potentialthroughput efficiency advantages of the hybrid ATM/TDM systems cannot berealized so long as false-positive trouble flags mask the true natureand location of trouble spots in a network.

Even for purely TDM payloads, the conventional reporting of transmissionerrors as “errored seconds”, does not accurately reflect the conditionof the data actually received. The errored seconds are measured usingparity bit (PB) or cyclic redundancy check (CRC) analysis, but they aredetermined using a fixed frame clock. In this time-locked conventionalerror reporting scheme, out-of-frame format errors (OOF) preempt theresults of parity and CRC tests, that is, every frame transmitted whilean OOF condition exists is a continuation of the transmission error,regardless of the accuracy of the data within that frame. When both thePB and CRC errors and the OOF errors are reported together as a totallength of time, the entire time elapsed before the expected frame formatreappears with the correct intra-frame parity or redundancy, is reportedin those “errored” or “OOF” seconds.

Conventional TDM service profiles reporting OOF “errored seconds” arecompiled by terminal equipment such as the DDM-1000 or the DCS 3/1, oran interface unit (IU) that is an adjunct to such terminal equipment.

Over-reporting of transmission errors is particularly serious for thedigital service providers (DSPs) who are subject to tariff regulations.Often times the detection of transmission errors fails to distinguishbetween the types of errors. This distinction could be important since aDSP must be prepared to defend its performance as a licensee by provingthe quality of their service and the reliability of the service thatthey have provided to the public under their operating licenses.

Greater accuracy in quantifying data losses, and greater detailreporting the occurrence of events that disrupt TDM communications, isespecially important for trouble analysis in long-distance circuits. Thecomplex interactions that occur when the service provided to a customerinvolves multiple companies, during “bridge and roll” re-direction ofcalls, for example, are accompanied by an increase in the incidence ofabnormal service interruptions of unknown origin.

In hybrid ATM/TDM networks voice traffic is, in part, carried onasynchronous packet-switched circuit. Terminal adapters (TA), operatingin the ATM domain reformat the payload received from conventional TDMnetworks into packets. In the ATM domain, voice and data are entirelyrecoverable from TDM inputs having OOF errors of less than a frame.Unfortunately, using standard TDM error criteria that slight OOF timingerror will be reported as a serious outage until framing is reset.OOF-based error reporting has been a blunt instrument for analyzingtrouble in conventional TDM payloads. In the new hybrid ATM/TDM networksthe instrument is even less useful as it swamps the TDM parity andredundancy test and other measures that reflect actual signaldegradation at the interface between TDM and ATM systems.

The data rate at the payload inputs to ATM networks need not be fixedand the relative packet timing across an ATM call is highly variable.There are three principal sources of delay in ATM communications:queuing delays in the packet switches; digital voice compressioncoding/decoding delays; and voice-packet assembly delays. In particular,the quality of a voice signal transmitted across an ATM network issensitive to round-trip delay and packet-order error, but not to“framing errors” or even individual dropped packets. Thus, the TerminalAdapters on the boundary between ATM and TDM networks that report OOFincidents as call outages do not accurately describe the frequency orextent of any impairment of the voice signal received by the ATM portionof the network, nor by the customers using the hybrid networks.

The present invention provides constant-frame error detection withsynchronization criteria to provide a more precise signature of acircuit impairment.

SUMMARY OF THE INVENTION

In accordance with the present invention, a sub-second circuitimpairment and framing-error profile is produced for a framed signal bya profiler having a bit counter, a frame pattern detector, a synccircuit and a profile extractor. Each time the frame pattern detectordetects a frame in the framed signal, it produces a detected framesignal that resets the bit counter. The bit counter then begins to countthe number of bits in the framed signal again. When the count in the bitcounter reaches the number of bits expected in a frame of the framedsignal it produces an estimated frame signal. If the estimated framesignal and the detected frame signal do not coincide with each other, aframing-error signal is produced.

Preferably a default frame signal is generated by a sync circuit tomaintain accurate synchronization of detector and received signal whenit happens that no bits are detected at the time when a detected framesignal is expected and at a time period calculated as a running averageof the time between recent detected-frame signals has also expiredwithout either a detected frame or an estimated frame signal. Becausethis pseudo-synchronous default signal does not coincide with a detectedframe signal, a framing-error signal is produced.

In one embodiment, the detected frame signal and the bit count expectedin each frame are also used to test data integrity. If the data (that isthe number of bits expected in a frame after each detected frame begins)fails the test, a data error signal is produced.

Preferably, the timing of each incidence of an error signal is recordedby an identifier and that identifier is stored in a profile. When theerror signal is a data error signal, the timing is preferably identifiedby a time-stamp value.

The output of the device of the present invention is a binary codemessage that establishes the format of the “profile.” This message canbe truncated when no impairments are detected.

The invention provides a compact sub-second error profile capable ofresolving multiple sub-second error events so that the operation ofhigh-speed signals, such as those in T1 and T3 networks, and synchronousoptical networks (SONET) and their interactions with signals in othernetworks, can be accurately described, even when the T-carrier signalgoes OOF. Sub-second accuracy permits trouble profiles built frombit-error data to be economically communicated across networks andanalyzed to improve transmission quality and reliability. It alsopermits the technician to distinguish OOF timing errors from dropout ornoise events causing data corruption. This sub-second accuracy isimportant for defending service reliability in ATM/TDM networks. In TDMnetworks it is particularly important where customer service,end-to-end, requires the facilities of multiple service providers. Theresulting increase in abnormal disruptions from “unknown” sourcesrequires detailed failure analysis to assure proper accountability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a T-carrier network on a SONET backbone withanalog and ATM sub-networks, having transmission-error detection inaccordance with the present invention.

FIG. 2 is a schematic diagram of sub-second error detection apparatus inaccordance with the present invention.

FIG. 3 is a schematic timing diagram for the apparatus of FIG. 2 showingsynchronous constant-frame error detection in a period when the signalis OOF and pseudo-synchronous constant-frame error detection during anoutage, when frame format information is absent from the signal.

FIG. 4 illustrates a flow chart with steps for detecting sub-seconderrors in accordance with the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

With reference first to FIG. 1, SONET transmission is implemented in anoptical-fiber long-distance backbone network 10 for T-carrier frameddata and voice payloads received from T3 lines through respectivedigital access and cross-connect systems (DACS) at the edge nodes 12 ofthe network 10. The T3 lines are multiplexed to other T3 nodes 13 and toT1 lines in local networks, which may serve analog customer terminals 14through conventional 5ESS switches 14 a as well as ATM customerterminals 16 through respective terminal adapters (TA) 16 a.

Transmission quality in the SONET network 10 is monitored at a signaltransfer point (STP) 17. The multiplexers 18 at the T3/T1 and T1/T0drops have profilers 20 that assemble sub-second framing-error anddata-error profiles for identifying transmission problems. Sub-secondframing-error profiles are also assembled by profilers 20 a in the edgenodes 12 of the SONET network 10. These profilers 20 a may be in theDACS itself, or in an interface unit (IU) associated with the DACS. Thecompressed frame data provided by these frame-error profiles is used foranalyzing the source of such trouble, as well as generating error-ratestatistics. The profiles provide both errors-per second values andsub-second framing profiles, rather than just OOF-seconds statistics,but they report only error incidents, thereby minimizing theadministrative bandwidth required by the network.

Circuits in the SONET network that have excessive frame-error rates, orare implicated in customer reports of noise on the line, calls droppedand call completion failures, are trouble-flagged by thenetwork-supervisor processor 22 which could be associated with STP 17.Call routing may by-pass the flagged circuits until their statistics areanalyzed and any trouble indicated therein is cleared. Certainly callsrequiring “guaranteed” classes of service, such as, for example, the“800” calls made by airlines' customers, will be routed around thetrouble flags if at all possible.

As shown in FIG. 2 bits in the digital signal received by a multiplexer18, for example, are detected and regenerated by signal conditioningcircuit 30 at the input to the profiler 20. The detected bit stream issupplied to a counter 32 and a signal framer 34. The signal framer 34advances the bit stream through a register 34 a one bit at a time andcompares the bits in the register against a frame-format template 34 beach time the bits advance. The profiler may be adapted to a variety ofT-framed telecommunications formats by retrieving a respective frameformat template, having an accompanying set of frame and bit timingstandards, from the template storage 34 c. A template that is compatiblewith the telecommunications format of the signal applied to the “TDM IN”input of the profiler 20 is selected. Each selected template 34 brepresents the relative locations and the values of the frame bits forthe T-framed signal format used by the TDM signal received at the input.

The device requires that it be synchronized to the incoming signal for atime so that the Bit Counter/Holdover Sync section can provide ESTIMATEDFRAME signals at the correct time.

For example, in T1 service, transmitted at 1.544 Mbits/sec., each T1frame uses 192 databits+1 frame bit to carry 24 T0 channels. However,the “SF” T1 format standard uses a 12-bit frame sequence “1000 11011100” so that an “in-frame” determination is actually made for groups oftwelve SF standard T1 frames. The extended SF standard (ESF) uses onlysix bits “001011” for 24 T1 frames, reserving 18 bits for in-band systemadministration signaling. SF-standard OOF frame errors are flagged when2 consecutive frame-bit errors are detected. ESF-standard OOF frameerrors are flagged when 4 consecutive frame-bit errors are detected.

In T3 service, transmitted at 44.736 Mbits/sec., the T3 “ASYNC (M13)” or“CBIT” format standard uses a more complex 31-bit frame sequence that isrepeated for redundancy “X1001X110110011001m1001M1001M1001”. In this T3frame sequence, X is a control bit that can have a value of either “1”or “0” and “M” and “m” are frame alignment bits having values of “1” and“0” respectively. T3-standard frame errors are flagged when {fraction(3/16)} consecutive frame-bit errors or 2 out of 3 consecutive alignmentbit errors are detected.

In SONET service, an STS1 transmitted at 51.84 Mbits/sec., carriesframing information as overhead and the framing sequence is a wordhaving two consecutive bytes “A1” and “A2”, where the expected over-allpattern is “11110110 0010100” (F628 Hex) bytes. SONET-standard frameerrors are flagged when 4 consecutive framing words are erroneous. Inthis and in the other types of service, each errored frame sequenceevent or pseudo-synchronous event is recorded in a framing-errorprofile.

When the comparator 34 d detects a coincidence between the framing bitvalues represented by the selected template 34 b and the bits in theregister 34 a, the output of the comparator 34 d goes high. The outputof this comparator 34 d is supplied to the first input of the “OOF” NANDgate 36.

This device requires synchronization of the valid frame signal and themonitored signal. This can be done using counter circuit 40.

The bits forwarded to the signal framer 34 are also received by the bitcounter 32 that is connected in parallel with the signal framer 34. Thebit counter 32 is reset whenever the bit counter 32 advances to “N” orthe output of the “CLOCK” AND gate 38 goes high. When the counter 32advances to the value “N”, that is a number of bits expected in a framefor the format of the selected template 34 b selected from templatestorage 34 c, a positive input is supplied to the second input of the“OOF” NAND gate 36. A framing-error incident value of “1” will berecorded in the shift register 44 each time an “ESTIMATED FRAME” pulseis generated, unless the “ESTIMATED FRAME” pulse coincides with a“DETECTED FRAME” pulse at the “OOF” NAND gate 36. If those pulsescoincide, a null framing-error value of “0” is output by the NAND gate36.

Before the bit counter 32 advances to the value “N” the second input ofthe “OOF” NAND gate 36 will go high if both gates of the “CLOCK” ANDgate 38 go high. The first input of “CLOCK” AND gate 38 goes high whenthe clock counter 40 advances to a numerical value greater than “N”before it is reset by the comparator 34 d, as described above withreference to the bit counter 32 and the same numerical value “N”. Thesecond gate of the “CLOCK” AND gate 38 goes high when the holdover syncgoes high the holdover sync which keeps a running average of theintervals between “RESET” pulses that are within a given range on eitherside of an interval value representing the standard nominal frame rateof the format of the selected template 34 b. The clock counter 40 isadvanced at a frequency representing a standard nominal bit rate for theformat of the selected template 34 b. Thus, the output of the “CLOCK”AND gate 38 goes high only after both the nominal frame interval and theholdover-sync interval have been exceeded before a “RESET” pulse isreceived from the comparator 34 d.

This “CLOCK” AND gate 38 provides the “flywheel” that maintains thesub-second resolution of the frame profile when the telecommunicationssignal applied to the “TDM N” input to the profiler is interrupted,halting the bit counter 32. The flywheel module is used to provide anEstimated Frame pulse at the regular intervals that would occur for anormal, unerrored, stream of data.

In sum, the constant-frame trigger signal provided by the DEFAULT FRAME”pulse in periods when the telecommunications signal is quiescent, and bythe “DEFAULT FRAME” pulse in cooperation with the bit counter 32 whenthe signal is corrupt, provide pseudo-synchronous constant-frame errordetection in the absence of a “DETECTED FRAME” pulse.

The shift register 44 clocks out an n×1 array of the framing-errorvalues when the register 44 is full and adds each n×1 array 46 to an n×Tarray which is discarded if it is not needed for on-going performanceaudit or maintenance work. The n×1 array 46 is also filtered to producea framing-error profile 48 providing a time-stamp entry 50 for eachframing-error incident value “1” in the n×T array 46. A frame-stampentry which is incremented 52 for each value in the n×1 array 46 mayalso be provided for each of the framing-error incident values “1” torecord the relative position of the errored frame among the other framesin the n×T array. This will occur particularly in circumstances whereframe intervals in the telecommunications signal are highly variable.

FIG. 3 illustrates the derivation of a “time-stamped” framing errorprofile from transient events causing framing errors. The time-stampvalues may be accumulated by a profiler 20 a for a convenient period oftime before being transmitted to the supervisory processor 22 in aburst, as an array filled with time-stamp values, to conserve bandwidth.

The “DETECTED FRAME” pulse produced by a profiler 20, such as the one inthe multiplexer 18 nearest to the cut “X” in the T3 cable shown in FIG.1, may also be extended to a data-integrity profiling circuit, shown inFIG. 2 to evaluate signal corruption as well as frame timing. The n×1arrays of frame-by-frame data-integrity values “0” and “1” would then befiltered and all integrity-error values “1” would be time stamped toproduce a data error profile of events that can be time-linked to eventsreflected in the framing-error profile. The series of frames representedby the “DETECTED FRAME” pulses may be discontinuous, unlike the framingerror profile. Thus, time stamps rather than frame stamps must be usedto coordinate data and framing error profiles, and the framing errorprofiles of different links in a circuit or different circuits withinthe sample physical trunk. Similarly, the “DETECTED FRAME” pulse may beused as a trigger signal for synchronous, constant-frame error detectionin an OOF signal using the PB or CRC bit test protocols, or partychecking, despite the existence of an OOF condition.

The trouble profiles compiled in accordance with present invention,using synchronous constant frames, and pseudo-synchronous frames whenframe sequences cannot be detected in the data, particularly whentransmission is interrupted and the carrier is temporarily absent, areaccurate down to the millisecond. Furthermore, the present inventionprotects network integrity by permitting the cause of outages to beaccurately traced to complex network interactions, by distinguishingdelays and disruptions that cause framing errors from data-corruptingevents which occur independent of frame-format timing.

For example, in FIG. 1, the cross-connect changes required for thebridge-and roll redirection of calls through alternate node “A” 13around the cut “X” in the T3 cable will trigger OOF events all along thepaths of the affected call circuits. However, the profilers 20 in themultiplexers 18 serving the 5ESS switch 14 a and the ATM terminaladapter 16 a can verify that the lost-data complaints from its datacustomers, and signal quality trouble experienced by voice customerscoincided precisely with the cable-cut event affecting the T3 line. Thisdistinguishes that event from events local to the T1 or SONET lines.

The profiler 20 in the multiplexer 18 serving the analog line throughthe 5ESS switch 14 a can also demonstrate that signal-quality troublesreported by the voice-grade customers 14 was caused by the T3framing-error outage. Furthermore, by selectively using the dataintegrity checking, the profiler 20 can also determine the sources ofnoise events that appear on local voice-grade lines apart from reportedcircuit outage and delay events, particularly noise events that occurwhile framing is within specification, is to from the T1 or analoglines, thus authoritatively resolving system-integration accountabilityissues.

FIG. 4 is a flow chart with steps for detecting sub-second errors inaccordance with the present invention. These steps may be implemented,for example, as a computer program or as computer hardware usingwell-known signal processing techniques. If implemented in software, thecomputer program instructions are stored in computer readable memory,such as Read-Only Memory (ROM), Random Access Memory (RAM), magneticdisk (e.g, 3.5″ diskette or hard drive), optical disk (e.g., CD-ROM) andso forth. In accordance with one embodiment of the present invention,these steps are implemented by the apparatus of FIG. 2.

In step 402, the apparatus detects a frame from within a T-framed inputsignal.

In step 404, the apparatus compares the detected frame with a frameformat template which is compatible with the frame format of the inputsignal.

In step 406, the apparatus determines whether the bits of the detectedframe match the bits of the frame format template. If the bits match,the apparatus proceeds to step 408 and then returns to step 402 todetect the next frame in the input signal. If the bits do not match, theapparatus proceeds to step 410 and then returns to step 402 to detectthe next frame in the input signal.

In step 408, the apparatus generates a “valid frame” output message at arate which is synchronized with the expected frame rate of the signalinput to the apparatus. As described above with respect to FIG. 3, thisoutput message might be a “0” bit.

In step 410, the apparatus generates an “invalid frame” output messageat a rate which is synchronized with the expected frame rate of thesignal input to the apparatus. As described above with respect to FIG.3, this output message might be a “1” bit.

After implementing the steps described above with respect to FIG. 4, thepresent invention generates an output signal comprising a series of bitswhich depict the integrity of the input signal at the frame level as afunction of time. The bit rate of this output signal is synchronizedwith the expected frame rate of the input signal. Thus, a profile of thetelecommunications signal is provided.

The method and apparatus of the present invention has been describedwith particular reference to presently preferred embodiments thereof.However, it will be apparent to one skilled in the art thatmodifications and variations are possible within the spirit and scope ofthe invention. Furthermore, this error-detection circuit has beendescribed in a schematic fashion, to assure that the one skilled in theart can make and use the method and apparatus of the present inventionwithout undue experimentation. The functional block diagram in FIG. 2and the output shown in FIG. 3 may be physically implemented by varioussuitable devices and formats already known in the art.

What is claimed is:
 1. A method of assembling a transmission performanceprofile for a circuit having multiple links in a network having aplurality of circuits and at least one node having an administrativemodule which provides error detection for at least one circuit, themethod comprising the steps of: detecting a frame within the inputsignal on a link; providing a frame format template compatible with theparticular type of input signal, the frame format template used togenerate an estimated frame signal; comparing the detected frame signalto the estimated frame signal to determine the presence of an error whenthe estimated frame signal does not coincide with the detected framesignal; generating a default frame signal using a synchronizationcircuit to maintain accurate synchronization between detection andreceived signal; and generating the transmission performance profileoutput signal comprising a series of bits depicting the integrity of theinput signal at the frame level as a function of time, the output signalsynchronized with the default frame signal.
 2. The method as defined inclaim 1 wherein the estimated frame signal is generated by a bit counterthat is reset when a new frame is received.
 3. Apparatus for flaggingtroubled circuits in an intelligent telecommunications network having aplurality of circuits, and a signaling network including a networksupervisory unit which stores trouble flags for circuits needingremedial action and at least one signal transfer point (STP) having anadministrative module that provides error date for a least one circuit,said apparatus comprising: means for detecting a frame within the inputsignal on a link; means for providing a frame format template compatiblewith the particular type of input signal, the frame format template usedto generate an estimated frame signal; means for comparing the detectedframe signal to the estimated frame signal to determine the presence ofan error when the estimated frame signal does not coincide with thedetected frame signal; means for generating a default frame signal usinga synchronization circuit to maintain accurate synchronization betweendetection and received signal; and means for generating the transmissionperformance profile output signal comprising a series of bits depictingthe integrity of the input signal at the frame level as a function oftime, the output signal synchronized with the default frame signal. 4.The apparatus as defined in claim 3 wherein the means for providing aframe format template include a bit counter that is reset when a newframe is received.